This is a randomized, partially-blinded trial for ACTG 175 participants who are on combination therapy at the time of enrollment into ACTG 303. Patients will be randomized to one of the following treatment arms and followed for 48 weeks: 1) ZDV+blinded ddI+ blinded 3TC placebo, or ZDV+blinded ddC+blinded 3TC placebo; 2) ZDV+blinded ddI+ +blinded 3TC, or ZDV+blinded ddC+blinded 3TC; 3)ZDV+blinded ddI placebo+blinded 3TC, or ZDV+blinded ddC placebo+blinded 3TC. This study seeks to examine virologic responses in subjects on prior nucleoside analog combination therapy with differential risks of disease progression. The objective will be to determine if viral clearance rates differ in the high-risk and low-risk groups, and to relate the immediate virologic responses to the mid-and longer-term responses. The practical question of the potential utility of 3TC, either as part of a three drug regimen or in combination with ZDV in patients with extensive prior ZDV/ddI or ZDV/ddC exposure, will be explored. This study is closed to enrollment 1/31/96.